Temperatures in the nozzle section of a gas turbine engine generally exceed 500.degree. C. In order to minimize the operating temperature of the structural components in the nozzle section, cooling air is typically forced over the components. An example is the hot section nozzle inserts which are circumscribed by the nozzle wall of a gas turbine. Under some circumstances, the flow rate of air over the nozzle inserts can be reduced, resulting in a higher operating temperature for the nozzle inserts and a higher temperature for the cooling air downstream of the nozzle inserts. The operating temperature of the nozzle inserts is determined in part by radiative heat transfer through the static air gap between the inner surfaces of the nozzle walls and the outer surfaces of the nozzle inserts. The inserts are typically made from a superalloy, such that their emissivity is high, thus promoting higher operating temperatures as a result of absorption of radiative thermal energy from the nozzle walls.
Various reflective coatings have been proposed in the past for the purpose of forming adherent heat shields on components which are subjected to thermal radiation. Such reflective coatings have often been a noble metal coating, such as platinum or gold, though other highly reflective materials have also been suggested. As a reflective coating, such heat shields are capable of reflecting most of the thermal radiation which is incident on the heat shield.
However, it has been determined that suitably reflective materials for use as a heat shield for nozzle inserts are unable to perform satisfactorily at the elevated temperatures sustained within the nozzle section of a gas turbine engine. More specifically, the reflectivity of such coatings significantly degrades at the elevated service temperatures of articles such as nozzle inserts, as a result of some constituents of the underlying substrate having a tendency to diffuse out into the coating when exposed to sufficiently high temperatures.
Accordingly, it would be desirable to provide a heat shield whose reflectivity is not degraded at elevated temperatures, particularly on the order of those experienced by hot section nozzle inserts of a gas turbine engine, such that the heat shield is able to effectively reflect a majority of the thermal radiation which is incident on the heat shield at such elevated temperatures.